The invention relates to an internal combustion engine having a multi-link crankshaft drive, wherein the multi-link crankshaft drive includes a plurality of coupling elements rotatably mounted on crankpins of a crankshaft and a plurality of articulated connecting rods rotatably mounted on crankpins of an eccentric shaft, wherein each of the coupling elements is pivotally connected to a piston connecting rod of a piston of the internal combustion engine and to one of the articulated connecting rods.
Internal combustion engines of the aforementioned type are known in the art. They include the eccentric shaft, which is coupled to the crankshaft via the multi-link crankshaft drive and is thus driven by the internal combustion engine or the crankshaft. The multi-link crankshaft drive has a number of coupling elements corresponding to the number of pistons of the internal combustion engine, which are each rotatably supported on the crankpin of the crankshaft and have two arms projecting on opposite sides of the crankshaft and having a pivoting joint at their end. One of the pivoting joints is used to pivotally connect with the piston connecting rod which connects one of the pistons of the internal combustion engine to the crank shaft via the coupling element. Another of the pivot joints is used to pivotally connect with the so-called articulated connecting rod which is supported with its other end rotatably on the crankpin of the eccentric shaft.
Similar to a conventional internal combustion engines without an eccentric shaft coupled to the crankshaft via a multi-link crankshaft drive, inertia forces of the first and second order are also generated in the internal combustion engines of the aforementioned type, which are caused by oscillating masses and which change with the crank angle of the crankshaft. To achieve a desired smooth running and to reduce noise, these inertial forces must be compensated as much as possible. While the first-order inertial forces can be compensated with counterweights on the crankshaft having a certain arrangement and a certain weight as well as with a certain crankshaft cranking sequence, the second order inertia forces are often compensated in conventional internal combustion engines by using two counter-rotating balance shafts, which are driven at twice the rotational speed of the crankshaft.
More specifically, the oscillating masses cause in all internal combustion engines free first-order and second-order inertia forces that vary with the crank angle of the crankshaft. While first-order inertia forces are balanced by the counterweights on the crankshaft and the crankshaft cranking sequence, the free second-order inertia forces cannot be fully compensated in conventional internal combustion engines having a multi-link crankshaft drive. For this reason, such internal combustion engines are inferior with regard to the smoothness or refinement of internal combustion engines lacking a multi-link crankshaft drive where second order inertia forces are often the compensated by using the two counter-rotating balance shafts. However, this measure cannot be easily transferred to internal combustion engines having a multi-link crankshaft drive, because the resulting inertia forces are, on the one hand, not purely oscillatory, but rather are rotational, and on the other, the friction losses of the multi-link crankshaft drive itself are already higher than the friction losses of conventional internal combustion engines and would increase to an unacceptable level because of the additional frictional losses of an additional balance shaft. However, even in internal combustion engines of the aforementioned type, a far-reaching compensation of the inertial forces can be attained with specific designs of the balance shaft.
Once the forces of inertia of the engine are almost balanced, the mass torques around the crankshaft axis, i.e. in the longitudinal direction of the internal combustion engine, greatly affect the engine acoustics. A total alternating torque Mw of the internal combustion engine about the longitudinal crankshaft axis is essentially composed of a gas exchange torque MGW, the mass alternating torque MMW and the balancing torque MA. In a four-cylinder in-line combustion engine all these components operate in the second order. The gas exchange torque is hereby produced by the gas forces tangentially introduced into the crankshaft and is thus the reaction torque, which is produced on the crankshaft when the internal combustion engine outputs a net torque. The mass alternating torque is generated by the support of the inertia forces of the multi-link crank drive on the crankshaft. The balancing torque in conventional multi-link crankshaft drives results from the vertical offset, which is present between two optional balance shafts. By selective design of this height offset, the total alternating torque can be reduced in certain operating ranges, i.e. speed and/or load ranges, thus improving the engine acoustics.
The multi-link crankshaft drive is preferably configured to have a second-order inertia force loop resembling a circle. The inertia force loop can be divided into a part rotating in the direction of rotation of crankshaft and a counter-rotating part. When the multi-link crankshaft drive has the circle-like second-order inertia force loop, at least the more dominant of the two parts, namely the part rotating in the direction of rotation of crankshaft can be compensated with only a single balance shaft that rotates at twice the rotational speed of the crankshaft. The usually considerably smaller, counter-rotating second order inertia force part which rotates in the opposite direction as the crankshaft inertia component could also be fully compensated by a second balance shaft. However, this inertia force part typically has a smaller inertia force amplitude in the second order than a mechanical valve train. Therefore, the second balance shaft can often be eliminated.